Isolation trench fill using oxide liner and nitride etch back technique with dual trench depth capability

ABSTRACT

An oxide layer is formed over a substrate having a smaller isolation trench and a large isolation trench. A nitride layer is formed over the oxide layer such that it completely fills the smaller isolation trench and lines the larger isolation trench. The nitride layer is etched back to form a recess in the nitride layer in the smaller isolation trench while at least a portion of the nitride layer lining the larger isolation trench is completely removed. A layer of HDP oxide is deposited over the substrate, completely filling the smaller and larger isolation trenches. The HDP oxide layer is planarized to the upper surface of the substrate. The deeper larger isolation trench may be formed by performing an etching step after the nitride layer has been etched back, prior to depositing HDP oxide.

FIELD OF THE INVENTION

The present invention relates to semiconductor processing and, inparticular, to a method of void-free filling of isolation trenches.

BACKGROUND OF THE INVENTION

With increasingly smaller dimension scaling in memory integrated circuit(IC) fabrication, filling deep isolation trenches, as may be used, forexample, in FLASH memory structures, without voids has become moredifficult. This is particularly true for isolation trenches that use anitride liner with an HDP oxide fill. The addition of a nitride linerprovides several benefits, including improved corner rounding at thebottom and sidewalls of the isolation trenches to decrease theoccurrence of voids, reduced stress adjacent the trench isolationstructure, and reduced electrical leakage. However, a nitride liner istypically a spin-on-dielectric (SOD). The SOD process leaves residualnitride materials in the vicinity of the active area. Nitrides at theactive areas may cause a shift in electrical parameters and devicereliability degradation.

Therefore, it is desirable to have a method of forming an isolationtrench, including deep isolation trenches, that can exploit theconformal properties of nitride film without leaving nitride materialsin the vicinity of the device's active areas.

BRIEF SUMMARY OF THE INVENTION

The invention provides a method of forming a void-free trench isolationstructure having a nitride liner and HDP oxide fill. In an exemplaryembodiment, an oxide layer is formed over a substrate having a smallerisolation trench and a larger isolation trench. For example, the smallerisolation trench may be for a memory array, while the larger isolationtrench may be used for isolating the memory array from peripherycircuitry. A nitride layer is formed over the oxide layer such that itcompletely fills the smaller isolation trench and lines the largerisolation trench. The nitride layer is etched back to form a recess inthe nitride layer in the small isolation trench while the nitride layerlining the large isolation trench is completely removed. A layer of HDPoxide is deposited over the substrate, completely filling the smallerand larger isolation trenches. The HDP oxide layer is planarized to theupper surface of the substrate. A deeper larger isolation trench may beformed by performing an etching step in the larger isolation trenchafter the nitride layer has been removed, prior to depositing the HDPoxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an trench isolation structure of thepresent invention at an early stage of fabrication;

FIG. 2 is a cross-sectional view of the trench isolation structure ofFIG. 1 at a subsequent stage of fabrication;

FIG. 3 is a cross-sectional view of the trench isolation structure ofFIG. 2 at a subsequent stage of fabrication;

FIG. 4 is a cross-sectional view of the trench isolation structure ofFIG. 3 at a subsequent stage of fabrication;

FIG. 5 is a cross-sectional view of the trench isolation structure ofFIG. 4 at a subsequent stage of fabrication of a first embodiment of thepresent invention;

FIG. 6 is a cross-sectional view of the trench isolation structure ofFIG. 5 at a subsequent stage of fabrication of the first embodiment ofthe present invention;

FIG. 7 is a cross-sectional view of the trench isolation structure ofFIG. 6 at a subsequent stage of fabrication of a second embodiment ofthe present invention;

FIG. 8 is a cross-sectional view of the trench isolation structure ofFIG. 7 at a subsequent stage of fabrication of the second embodiment ofthe present invention;

FIG. 9 is a cross-sectional view of the trench isolation structure ofFIG. 8 at a subsequent stage of fabrication of the second embodiment ofthe present invention;

FIG. 10 is a cross-sectional view of the trench isolation structure ofFIG. 9 at a subsequent stage of fabrication of the second embodiment ofthe present invention;

FIG. 11 is a diagram of an exemplary memory array employing anembodiment of the present invention; and

FIG. 12 is a block diagram of a processor system utilizing componentsconstructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to variousspecific exemplary embodiments in which the invention may be practiced.These embodiments are described with sufficient detail to enable thoseskilled in the art to practice the invention, and it is to be understoodthat other embodiments may be employed, and that structural, logical,and electrical changes may be made.

The term “substrate” used in the following description may include anysemiconductor-based structure that has a semiconductor surface.Substrate must be understood to include silicon, silicon-on insulator(SOI), silicon-on sapphire (SOS), doped and undoped semiconductors,epitaxial layers of silicon supported by a base semiconductorfoundation, and other semiconductor structures. The semiconductor neednot be silicon-based. The semiconductor could be silicon-germanium,germanium, or gallium arsenide.

Referring now to the drawings, where like elements are designated bylike reference numerals, FIGS. 1-10 illustrate a method of formingisolation trenches according to exemplary embodiments of the invention.FIG. 1 illustrates a semiconductor substrate 10 within which isolationtrenches 21 and 22 have been formed by conventional methods. A padnitride layer 20 lies over the top surface of the semiconductorsubstrate 10 and the trenches are formed into layer 20 and theunderlying substrate 10. Optionally, there may be a poly-amorphoussilicon layer between the pad nitride and the surface of thesemiconductor substrate 10 (not shown). Isolation trenches 21 and 22have the substantially the same depth in the semiconductor substrate 10,however isolation trench 21 is smaller (i.e., less volume and smallerwidth) than larger isolation trench 22. Small isolation trench 21 may beformed, for example, for an isolating structure in a memory array, whilelarger isolation trench 22 may be formed, for example, for an isolatingstructure in an adjacent periphery circuit.

Referring now to FIG. 2, subsequent to the formation of the trenches 21,22, an oxide liner 30 is formed over the substrate 10 to line thetrenches 21, 22. The oxide liner 30 is deposited to isolate the activeareas formed in the substrate 10 from nitrides that are deposited insubsequent processing steps. It should be noted that the sizes of thetrenches are not of particular importance. However, the invention may beemployed where one trench is smaller, relative to the larger trench,such that subsequent nitride deposition fills the smaller trench butonly lines the bottom and sidewalls of the larger trench.

As shown in FIG. 3, a nitride layer 40 is deposited over the substrate10, filling the small isolation trench 21 while only lining the largeisolation trench 22. The nitride layer 40 aids in smoothing out thecorners in the bottom of the trenches 21, 22 and in reducing the amountof stress in the dielectric used to subsequently fill in the trenches21, 22. A nitride etch-back process is next performed to remove all, ora portion, of nitride material from the upper surface of the FIG. 3structure and from the large isolation trench 22 and recess the nitridematerial in the small isolation trench 21 to a level below the topsurface of the substrate 10, as shown in FIG. 4. The remaining nitridelayer 41 is recessed to a depth sufficient to separate nitride materialin remaining nitride layer 41 from any transistor gates in the finalmemory array in the region of the small isolation trench 21. Forexample, for a FLASH memory array, the nitride material 41 is separatedfrom any transistor floating gates which may be formed in active areasisolated by the trench 21.

FIG. 5 shows a subsequent stage of fabrication where an HDP oxidematerial layer 50 is deposited over the substrate 10. The HDP oxidelayer 50 fills the small isolation trench 21 and large isolation trench22. A planarization process, such as chemical mechanical planarization(CMP), is then used to planarize the layers on the surface of thesubstrate 10, as shown in FIG. 6. The resulting structure is a void-freesmaller trench isolation structure having a nitride liner and HDP oxidefill, without having nitride residue surrounding the trench isolationstructure or nitride residue in peripheral isolation structures adjacentto the trench isolation structure.

In another embodiment, a deeper trench may be formed in the peripheryarea, if desired. Initial steps of processing are used as shown in FIGS.1-4. After the nitride layer 41 is recessed to the desired depth in thesmall isolation trench 21 and removed from the larger trench 22 andupper surfaces of the FIG. 3 structure, the oxide liner layer 30 isetched at the bottom of trench 22, as shown in FIG. 7. A reactive-ionetch process is used to etch back oxide liner layer 30, leaving an oxideliner 33 in the small isolation trench 21 and oxide spacers 34 a, 34 bon the sidewalls of the large isolation trench 22. The hashed lines inFIG. 7 illustrate the oxide liner 30 which is removed during theetching.

The substrate 10 then undergoes a selective silicon reactive-ion etchprocess, as shown in FIG. 8. The etch process forms a deep trench forthe larger isolation trench 23, having a bottom 23 a that is deeper intothe silicon substrate 10 than the bottom 21 a of the small isolationtrench 21.

Referring now to FIG. 9, an HDP oxide material layer 60 is depositedover the substrate 10 to fill the small isolation trench 21 and deepisolation trench 23. A planarization process, such as CMP, is then usedto planarize the surface of the substrate 10, as shown in FIG. 10.

The exemplary embodiments of the invention (described above) form largerand smaller void-free trench isolation structures, with nitride in thesmaller trenches but without nitride residue surrounding the smallertrench isolation structures which may degrade devices constructed in theactive areas of the substrate isolated by the smaller isolationtrenches.

Although the invention has been described with reference to theformation of only two trench isolation structures, the invention alsocontemplates the formation of a multitude of larger and smallerisolation structures, having various depths, and located at variouslocations on the substrate to isolate devices. Further, although theinvention has been described above with reference to a memory array andperiphery circuitry, the invention also has applicability to otherintegrated circuits. For example, the invention may be used in flashmemory with the smaller trenches isolating structures in the memoryarray and the larger trenches isolation structures elsewhere, such asthe periphery, but can be used in any integrated circuit device whereisolation is required.

FIG. 11 is a diagram of an exemplary FLASH memory device 416 employingan embodiment of the invention. Smaller trench isolation structuresconstructed in accordance with the invention may be formed to isolate amemory cell in the main memory array region 101 while a larger trenchisolation structure may be formed in accordance with the invention toisolate the main memory array from the row decode circuit, or periphery,region 102.

FIG. 12 is a block diagram of a processor system 400 utilizing a flashmemory device 416 constructed in accordance with the present invention.That is, the flash memory device 416 has cells separated by a trenchisolation region constructed in accordance with the invention. Theprocessor system 400 may be a computer system, a process control systemor any other system employing a processor and associated memory. Thesystem 400 includes a central processing unit (CPU) 402, e.g., amicroprocessor, that communicates with the flash memory 416 and an I/Odevice 408 over a bus 420. It must be noted that the bus 420 may be aseries of buses and bridges commonly used in a processor system, but forconvenience purposes only, the bus 420 has been illustrated as a singlebus. A second I/O device 410 is illustrated, but is not necessary topractice the invention. The processor system 400 also includes randomaccess memory (RAM) device 412 and may include a read-only memory (ROM)device (not shown), and peripheral devices such as a floppy disk drive404 and a compact disk (CD) ROM drive 406 that also communicate with theCPU 402 over the bus 420 as is well known in the art.

The above description and drawings are only to be consideredillustrative of exemplary embodiments, which achieve the features andadvantages of the invention. Modification and substitutions to specificprocess conditions and structures can be made without departing from thespirit and scope of the invention. Accordingly, the invention is not tobe considered as being limited by the foregoing description anddrawings, but is only limited by the scope of the appended claims.

1. A method of forming a trench isolation structure in an integratedcircuit comprising: forming a smaller isolation trench and a largerisolation trench in a semiconductor substrate, wherein said largerisolation trench has a greater width than said smaller isolation trench;forming an oxide layer over said substrate and on a surface of saidsmaller and larger isolation trenches; forming a nitride layer over saidoxide layer, said nitride layer completely filling said smallerisolation trench, but only lining said larger isolation trench; removinga portion of said nitride layer to form a recess in said nitride layerin said smaller isolation trench and to completely remove said nitridelayer lining for said larger isolation trench; and forming a layer ofhigh density plasma oxide over said substrate to fill said smaller andlarger isolation trenches.
 2. The method of claim 1, wherein saidnitride layer is removed such that an upper surface of said nitridelayer in said smaller isolation trench is below an upper surface of saidsemiconductor substrate.
 3. The method of claim 1, further comprisingthe act of performing an oxide spacer etch on said oxide layer afterremoving said portion of said nitride layer to etch back said oxidelayer, leaving an oxide liner in said smaller isolation trench and oxidespacers on sidewalls of said large isolation trench.
 4. The method ofclaim 3, further comprising the act of selectively etching said siliconsubstrate to form said larger isolation trench to be deeper in saidsubstrate than said smaller isolation trench after performing said oxidespacer etch.
 5. The method of claim 1, further comprising the act ofplanarizing said high density plasma oxide.
 6. A method of forming atrench isolation structure in an integrated circuit comprising: formingfirst and second isolation trenches in a semiconductor substrate;forming a nitride layer over said substrate such that said nitride layercompletely fills said first isolation trench while only lining saidsecond isolation trench; removing a portion of said nitride layer toform a recess in said nitride layer in said first isolation trench; andforming an insulating layer of material over said substrate to fill saidfirst and second isolation trenches.
 7. The method of claim 6, whereinan upper surface of said nitride layer in said first isolation trench isbelow an upper surface of said semiconductor substrate.
 8. The method ofclaim 6, further comprising the act of forming an oxide layer over saidsubstrate and on a surface of said first and second isolation trenchesprior to said act of forming a nitride layer.
 9. The method of claim 8,further comprising the act of etching said oxide layer after removingsaid portion of said nitride layer to form an isolating liner in saidfirst isolation trench and isolating spacers on sidewalls of said secondisolation trench.
 10. The method of claim 6, further comprising the actof selectively etching said silicon substrate to form a deeper secondisolation trench after performing said etching step on said firstisolating layer.
 11. The method of claim 6, wherein said first isolationtrench is narrower than said second isolation trench.
 12. The method ofclaim 6, wherein said insulating layer is an HDP oxide layer.
 13. Themethod of claim 6, further comprising the act of planarizing saidinsulating layer to an upper surface of said semiconductor substrate.14. A method of forming a trench isolation structure comprising: forminga first trench having a first width and being completely filled with atrench liner, a nitride material plug recessed from a top of the firsttrench, and a high density plasma oxide material; and forming a secondtrench having a second width that is different than said first width andbeing completely filled with said liner and high density plasma oxidematerial.
 15. The method of claim 14, wherein the act of forming a firsttrench includes forming a nitride layer over said liner and removing aportion of said nitride liner to form a recess in said trench.
 16. Themethod of claim 14, further comprising planarizing said high densityplasma oxide materials to an upper surface of a substrate.
 17. Themethod of claim 14, wherein said first trench is smaller than saidsecond trench.
 18. A method of forming a trench isolation structurecomprising: forming a larger-width trench and a smaller-width trench,both trenches having a trench liner; completely filling saidsmaller-width trench with nitride material while lining saidlarger-width trench with nitride material; performing a nitride etch inat least said smaller-width trench to recess a top surface of saidnitride in said smaller-width trench; and completely filling saidsmaller-width trench and said larger-width trench with oxide on top ofsaid nitride.
 19. The method of claim 18, further comprising the act ofetching said larger-width trench to be deeper than said smaller-widthtrench prior to filling said larger-width trench.